Since monomeric diisocyanates cannot be used as crosslinkers in polyurethane coating systems, owing to their volatility and toxicological properties, the general approach is to use the higher molecular mass derivatives, examples being those on a uretdione, isocyanurate, biuret, urethane or allophanate basis. An overview of these polyisocyanates and how to prepare them is given for example in J. Prakt. Chem./Chem. Ztg. 1994, 336, 185-200. In the field of lightfast paints and coatings it is normal to use polyisocyanates based on aliphatic and/or cycloaliphatic diisocyanates.
The oligomerization (normally dimerization or triimerization) of isocyanates to form uretdiones, isocyanurates or iminooxadiazinediones is a long-known method well established in practice for the modification of generally difunctional C1-C30 isocyanates of low molecular mass. Specifically for isocyanate dimerization, however, there have to date been only a few usable catalysts which possess high activity and selectivity and are also suitable for use on the industrial scale.
An overview of the industrially relevant dimerization processes of the prior art and of the catalysts or catalyst systems they employ is given in J. Prak. Chem. 336 (1994) 185-200. A disadvantage of the catalysts reported therein is their in some cases inadequate catalytic activity and poor selectivity towards dimerization; consequently there is a need for new, improved systems for industrial use in particular.
EP-A 45 995 describes the use of special peralkylated aminophosphines as catalysts for selectively dimerizing isophorone diisocyanate (IPDI) (trimer content <2% by weight). A substantial drawback of these compounds, however, is their oxidation sensitivity to phosphoramides (e.g. hexamethylphosphoramide (HMPA)), which possess a high carcinogenic potential, which is prohibitive for broad industrial use.
EP-A 317 744 describes a process for preparing linear (cyclo)aliphatic uretdiones by catalysis with 4-dimethylaminopyridines, such as 4-dimethylaminopyridine (4-DMAP). This process too delivers linear IPDI uretdiones virtually free from isocyanurate groups.
The catalysts both of EP-A 45 995 and of EP-A 317 744 exhibit only moderate catalytic activity and are inactive towards isocyanates whose NCO groups are attached exclusively to secondary carbon atoms (e.g. 4,4′-diisocyanatodicyclohexylmethane).
An improved catalytic activity is displayed by the azolate anions described in WO 02/92658, with cycloaliphatic diisocyanates in particular being converted to dimeric uretdiones with a high selectivity.
It was an object of the present invention, therefore, to provide new catalysts for dimerization which in addition to high selectivity also display a markedly improved catalytic activity and can be used on the industrial scale.